Resin materials having high refractive indices have high processability as compared with conventional glass materials, and are useful for optical materials that are intended to construct optical devices such as optical lenses, optical fibers and light waveguides. Among them, polycarbonates, polyurethanes, polyesters, polyamides, and the like are widely known to be resin materials having high refractive indices.
Since polycarbonates have satisfactory transparency and heat resistance, polycarbonates are frequently used in optical materials. However, even for aromatic polycarbonates, their refractive indices cannot be said to be sufficiently high, and there is a demand for the development of a resin for optical materials having a higher refractive index.
An example of the resin material having a high refractive index may be a polyester-based block copolymer containing a polyester block chain adopting a fluorene skeletal structure (see, Patent Literature (hereinafter, abbreviated as PTL) 1). Furthermore, a polyimide obtainable from a diamine compound having a naphthalene skeletal structure is known (see PTL 2). These copolymers are considered to be useful in materials for forming optical devices.